D_E1M6 loops improperly, the intro part (the brassy part) is also appended at the very end of the song, I came to rectify this for you.
Download . Open PK3 with SLADE, import into D_E1M6 and save archive.
Allow me to critique things so far:
E1M1: Erroneous end segment.
E1M6: Erroneous end segment, which makes it sound weird when it loops. I fixed it above for you.
E1M8/E3M4/E4M1: Interesting choice in place of the choir ahhs but the alternate does the job for me.
E2M2: Ocarina start instrument doesn't sound "windy" enough (IE SNES Doom's E2M2/E2M3)
E2M4: Sounds really strange using the pizzicato string in place of choir ahhs but the alternate version rectifies that. Tempo seemed a teensy bit above normal but I
fixed it (decreased tempo by 3%)
E2M6: Has erroneous end segment doubled so it sounds weird when it loops.
Corrected this as well.
E2M7: Has erroneous end segment at end.
E2M9/E3M1: Erroneous end segment.
E3M2: Erroneous end segment.
NOTE: These "end segment" issues I am finding makes looping tracks sound awkward or "off", BTW. I will make a PK3 archive with all the fixed tracks for you.
What OGG quality are you using, because I have been saving the files at default (quality 5) or at quality 6 in Audacity.
More critique to come (and fixes!)